Abstract:

The present invention provides high strength thick steel plate superior in
crack arrestability high in strength, free of deterioration of HAZ
toughness, and free of anisotropy, that steel plate containing, by mass
%, C: 0.03 to 0.15%, Si: 0.1 to 0.5%, Mn: 0.5 to 2.0%, P: ≦0.02%,
S: ≦0.01%, Al: 0.001 to 0.1%, Ti: 0.005 to 0.02%, Ni: 0.15 to 2%,
and N: 0.001 to 0.008% and having a balance of iron and unavoidable
impurities as chemical components, having a microstructure of a ferrite
and/or pearlite structure with bainite as a matrix phase, and having an
average circle equivalent diameter of crystal grains with a crystal
misorientation angle of 15° or more of 15 μm or less in the
regions of 10% of plate thickness from the front and rear surfaces and of
40 μm or less in the other region including the center part of plate
thickness.

Claims:

1. High strength thick steel plate containing, by mass %, C: 0.03 to
0.15%, Si: 0.1 to 0.5%, Mn: 0.5 to 2.0%, P: ≦0.02%, S:
≦0.01%, Al: 0.001 to 0.1%, Ti: 0.005 to 0.02%, Ni: 0.15 to 2%, and
N: 0.001 to 0.008% and having a balance of iron and unavoidable
impurities as chemical components, having a microstructure of a ferrite
and/or pearlite structure with bainite as a matrix phase, and having an
average circle equivalent diameter of crystal grains with a crystal
misorientation angle of 15.degree. or more of 15 μm or less in the
regions of 10% of plate thickness from the front and rear surfaces and of
40 μm or less in the other region including the center part of plate
thickness.

2. High strength thick steel plate superior in crack arrestability as set
forth in claim 1 characterized by further containing, by mass %, one or
more of Cu: 0.1 to 1%, Cr: 0.1 to 1%, Mo: 0.05 to 0.5%, Nb: 0.005 to
0.05%, V: 0.02 to 0.15%, and B: 0.0003 to 0.003% as chemical components.

3. High strength thick steel plate superior in crack arrestability as set
forth in claim 1 or 2 characterized by further containing, by mass %, one
or more of Ca: 0.0003 to 0.005%, Mg; 0.0003 to 0.005%, and REM: 0.0003 to
0.005% as chemical components.

4. High strength thick steel plate superior in crack arrestability as set
forth in claim 1 or 2 wherein {100} planes forming an angle of
.+-.15.degree. with respect to a plane vertical to loading direction have
an area ratio of 30% or less in said regions of 10% of plate thickness
from the front and rear surfaces.

5. High strength thick steel plate superior in crack arrestability as set
forth in claim 1 or 2 wherein said {100} planes forming an angle of
.+-.15.degree. with respect to the plane vertical to loading direction
have an area ratio of 15% or less in said regions including the center
part of plate thickness other than said regions of 10% of plate thickness
from the front and rear surfaces.

6. High strength thick steel plate superior in crack arrestability as set
forth in claim 1 or 2 wherein the plate thickness is 40 mm or more.

7. High strength thick steel plate superior in crack arrestability as set
forth in claim 1 or 2 wherein the yield stress is 390 MPa or more.

8. High strength thick steel plate superior in crack arrestability as set
forth in claim 3 wherein {100} planes forming an angle of .+-.15.degree.
with respect to a plane vertical to loading direction have an area ratio
of 30% or less in said regions of 10% of plate thickness from the front
and rear surfaces.

9. High strength thick steel plate superior in crack arrestability as set
forth in claim 3 wherein said {100} planes forming an angle of
.+-.15.degree. with respect to the plane vertical to loading direction
have an area ratio of 15% or less in said regions including the center
part of plate thickness other than said regions of 10% of plate thickness
from the front and rear surfaces

10. High strength thick steel plate superior in crack arrestability as set
forth in claim 4 wherein said {100} planes forming an angle of
.+-.15.degree. with respect to the plane vertical to loading direction
have an area ratio of 15% or less in said regions including the center
part of plate thickness other than said regions of 10% of plate thickness
from the front and rear surfaces.

Description:

[0002]Thick steel plate used for shipbuilding, construction, tanks, marine
structures, line pipe, and other structures are being required to exhibit
the ability to suppress propagation of brittle fractures, that is, crack
arrestability, in order to suppress the brittle fractures of such
structures. In recent years, along with the enlargement of structures,
high strength thick steel plate with a yield stress of 390 MPa to 500 MPa
and a plate thickness of 40 mm to 100 mm is being used in increasing
cases. However, in general, strength and plate thickness are
contradictory in the crack arrestability. The above crack arrestability
falls along with an increase in the strength and the plate thickness. For
this reason, technology for improving the crack arrestability in high
strength thick steel plate is desired.

[0003]As technology for improving the crack arrestability, for example,
the method of controlling the crystal grain size, the method of
controlling the brittle second phase, and the method of controlling the
texture are known.

[0004]As the method of controlling the crystal grain size, the technology
described in Japanese Patent Publication (A) No. 61-235534, Japanese
Patent Publication (A) No. 2003-221619, and Japanese Patent Publication
(A) No. 5-148542 is known. This uses ferrite as the matrix phase and
makes the ferrite finer so as to improve the crack arrestability.

[0005]Further, as the method of controlling the brittle second phase,
there is the technology described in Japanese Patent Publication (A) No.
59-49323. This disperses a fine brittle second phase (for example,
martensite) in the ferrite forming the matrix phase so as to cause fine
cracks in the brittle second phase at the front ends of the brittle
cracks and thereby release the stress conditions at the crack tips.

[0006]Further, as the method of controlling the texture, there is the
technology described in Japanese Patent Publication (A) No. 2002-241891.
This promotes the formation of a {211} plane texture parallel to the
rolled surface in ultralow carbon (C<0.003%) bainite single phase
steel.

[0007]However, these technologies have the following problems.

[0008]The technology of controlling the grain size uses soft ferrite as a
matrix phase, so obtaining a high strength thick steel plate is
difficult,

[0009]Further, with the technology of controlling the brittle second
phase, martensite is dispersed in the ferrite, so the crack initiation
property of the brittle fracture ends up remarkably deteriorating.

[0010]Further, since ferrite is used as the matrix phase, obtaining high
strength thick steel plate is difficult in the same way.

[0011]Further, in the technology for controlling the texture, ultralow
carbon steel is used and the structure is made a bainite single phase to
promote the formation of a uniform texture in the plate thickness
direction, so the crack arrestability cannot be remarkably improved.
Further, the load required for steelmaking for obtaining ultralow carbon
steel is extremely large.

DISCLOSURE OF THE INVENTION

[0012]The present invention was made in consideration of the above
situation and has as its object the ability to provide high strength
thick steel plate superior in crack arrestability which is high in
strength, free of deterioration of the HAZ (heat affected zone)
toughness, and free of anisotropy, at a low manufacturing cost.

[0013]To achieve the above object, the high strength thick steel plate
according to the present invention is as follows:

[0014](1) High strength thick steel plate containing, by mass %, C: 0.03
to 0.15%, Si: 0.1 to 0.5%, Mn: 0.5 to 2.0%, P: ≦0.02%, S:
≦0.01%, Al: 0.001 to 0.1%, Ti: 0.005 to 0.02%, Ni: 0.15 to 2%, and
N: 0.001 to 0.008% and having a balance of iron and unavoidable
impurities as chemical components, having a microstructure of a ferrite
and/or pearlite structure with bainite as a matrix phase, and having an
average circle equivalent diameter of crystal grains with a crystal
misorientation angle of 15° or more of 15 μm or less in the
regions of 10% of plate thickness from the front and rear surfaces and of
40 μm or less in the other region including the center part of plate
thickness.

[0015](2) High strength thick steel plate superior in crack arrestability
as set forth in (1) characterized by further containing, by mass %, one
or more of Cu: 0.1 to 1%, Cr: 0.1 to 1%, Mo: 0.05 to 0.5%, Nb: 0.005 to
0.05%, V; 0.02 to 0.15%, and B: 0.0003 to 0.003% as chemical components.

[0016](3) High strength thick steel plate superior in crack arrestability
as set forth in (1) or (2) characterized by further containing, by mass
%, one or more of Ca: 0.0003 to 0.005%, Mg: 0.0003 to 0.005%, and REM:
0.0003 to 0.005% as chemical components.

[0017](4) High strength thick steel plate superior in crack arrestability
as set forth in any one of (1) to (3) characterized in that {100} planes
forming an angle of ±15° with respect to a plane vertical to
loading direction have an area ratio of 30% or less in said regions of
10% of plate thickness from the front and rear surfaces.

[0018](5) High strength thick steel plate superior in crack arrestability
as set forth in any one of (1) to (4) characterized in that said {100}
planes forming an angle of ±15° with respect to the plane
vertical to loading direction have an area ratio of 15% or less in said
regions including the center part of plate thickness other than said
regions of 10% of plate thickness from the front and rear surfaces.

[0019](6) High strength thick steel plate superior in crack arrestability
as set forth in any one of (1) to (5) characterized in that the plate
thickness is 40 mm or more.

[0020](7) High strength thick steel plate superior in crack arrestability
as set forth in any one of (1) to (6) characterized in that the yield
stress is 390 MPa or more.

[0021]According to the present invention, the steel plate becomes
extremely superior in crack arrestability, high in strength even if thick
in plate thickness, and free of deterioration in HAZ toughness, so it
becomes possible to lower the cost and improve the safety of welded steel
structures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a view showing the relationship between the amount of
addition of Ni and the crystal grain size.

[0023]FIG. 2 is a view showing a grain boundary map obtained by
measurement by the EBSP method.

[0024]FIG. 3 is a view showing a {100} plane map obtained by measurement
by the EBSP method.

BEST MODE FOR CARRYING OUT THE INVENTION

[0025]Below, embodiments of the present invention will be explained. The
high strength thick steel plate according to the present embodiment has a
microstructure comprised of a ferrite and/or pearlite structure with
bainite as the matrix phase and is controlled in crystal grain size and
texture in the plate thickness direction so is improved in the crack
arrestability.

[0026]The reason for making bainite the matrix phase is to obtain steel
plate with a thick plate thickness and a high strength. With ferrite as
the matrix phase, obtaining such a steel plate is difficult. If making
bainite the matrix phase enables steel plate of the desired plate
thickness and strength to be obtained, the ferrite and/or pearlite may
also be made the second phase.

[0027]In general, the grain size of bainite depends on the grain size of
austenite before transformation to bainite. For this reason, making the
grain size of the bainite finer is difficult. As opposed to this, the
inventors engaged in intensive studies and as a result learned that by
making the amount of addition of Ni a suitable value, it is possible to
make the grain size of the bainite finer.

[0028]The graph of FIG. 1 shows the relationship between the amount of
addition of Ni and the average circle equivalent diameter of the crystal
grains having a crystal misorientation angle of 15° or more in a
bainite structure (crystal grain size) in the case of changing the
cooling rate after hot rolling to 5 to 30° C./s. The chemical
components other than Ni are, by mass %, C: 0.01%, Si: 0.2%, Mn: 1.3%, P:
0.005%, S: 0.003%, Al: 0.03%, Ti: 0.01%, and N: 0.003%. From this graph,
it reveals that if increasing the amount of Ni added, the crystal grains
become finer and further if increasing the cooling rate, the crystal
grains become finer.

[0029]The cooling rate of steel plate of a plate thickness of over 40 mm
is often about 30° C./s at the regions of 10% of plate thickness
from the front and rear surfaces of the steel plate (hereinafter referred
to as "the surface layer parts of the steel plate"). In this case, the
region other than the surface layer parts of the steel plate including
the center part of plate thickness (below, called the "center part of the
steel plate"), it is often about 50°C./s. The fact that when
making the amount of Ni added 0.15% or more at this cooling rate, the
crystal grain sizes at the surface layer parts of the steel plate and the
center part of the steel plate become 15 μm or less and 40 μm or
less can be read from FIG. 1.

[0030]Further, in this way, it was learned that when the crystal grain
size is 15 μm or less at the surface layer parts of the steel plate
and is 40 μm or less at the center part of the steel plate, a high
crack arrestability of a Kca at -10° C. of 170 MPam0.5 or
more is exhibited.

[0031]FIG. 2 is a grain boundary map showing the measurement results by
the EBSP method in thick steel plate of a plate thickness of 80 mm having
as chemical components, by mass %, C: 0.08%, Si: 0.2%, Mn: 1.1%, P:
0.005%, S: 0.005%, Al: 0.01%, Ti: 0.008%, Ni: 1.0%, N: 0.002%, Nb:
0.015%, B: 0.001%, and Ca: 0.001%. In the example shown in FIG. 2, the
crystal grain size is 6 μm at a portion positioned 5 mm below the
surface of the steel plate, 11 μm at a portion positioned at 1/4 the
plate thickness from the surface, and 18 μm at a portion positioned at
1/2 of the plate thickness. Thick steel plate having a crystal grain size
of 15 μm or less at the surface layer parts of the steel plate and of
40 μm or less at the center part of the steel plate exhibits a high
crack arrestability of a Kca at -10° C. of 200 MPam0.5.

[0032]The finer the crystal grain size, the better the crack
arrestability, but if considering the productivity, the lower limit of
the crystal grain size is preferably 3 μm at the surface layer parts
of the steel plate and 10 μm at the center part of the steel plate.

[0033]The reason why the crystal grain size becoming finer results in the
crack arrestability becoming improved in this way is as follows: At the
crystal grain boundary, the crystal orientation differs between adjoining
crystal grains, so at this part the direction in which the crack
propagates differs. For this reason, fracture-free regions occur. Due to
the fracture-free regions, the stress is shared and becomes crack closure
stress. Therefore, the driving force for crack propagation falls and the
crack arrestability is improved. Further, the fracture-free regions
finally fracture by ductile fracture, so the energy required for brittle
fracture is absorbed. For this reason, the crack arrestability is
improved.

[0034]In general, at the surface layer of thick steel plate, brittle
fracture does not easily occur and a ductile fracture region (shear lip)
easily forms. If the surface layer becomes finer grained and the
thickness of the finer grain layer becomes greater, the shear lip region
is enlarged. At the fracture-free region before formation of the shear
lip, the stress is shared and becomes crack closure stress. Further, the
energy required for brittle fracture is absorbed by formation of the
shear lip. For this reason, the crack arrestability is improved.

[0035]The reason for making the crystal misorientation angle with
adjoining grains 15° or more is that if less than 15°, the
crystal grain boundaries do not easily become resistance to propagation
of the brittle cracks and the above effect of improvement of the crack
arrestability is reduced. Further, the reason for making the crystal
grain size of the surface layer parts of the steel plate 15 μm or less
is that if over 15 μm, the toughness required for formation of a shear
lip cannot be obtained. The reason for making the crystal grain size of
the center part of the steel plate 40 μm or less is that if over 40
μm, the toughness falls, propagation of brittle cracks inside the
plate thickness becomes dominant, and the driving force for fractures at
the surface layer parts becomes larger, whereby shear lips become harder
to form.

[0036]On the other hand, the brittle cracks occurring at the steel plate
propagate along the cleavage plane of the {100} plane, so it is learned
that if a {100} plane texture develops at the plane vertical to the
loading direction, the effect of improvement of the crack arrestability
when controlling the crystal grain size in this way ends up being
reduced.

[0037]At this time, if the texture of the {100} plane forming an angle of
±15° with respect to the plane vertical to the loading
direction becomes, by area ratio, 30% or less at the regions of 10% plate
thickness from the front and rear surfaces (surface layer parts of the
steel plate), it is learned that the effect of improvement of the crack
arrestability due to the increased fineness of the crystal grain size can
be exhibited and a sufficient value of the crack arrestability is shown.
Further, at the region other than the surface layer parts of the steel
plate including the center part of plate thickness (center part of the
steel plate), it is learned that if making the area ratio of the texture
15% or less, the effect of improvement of the crack arrestability due to
the increased fineness of the crystal grain size can be exhibited and a
sufficient value of the crack arrestability is shown.

[0038]FIG. 3 is a map of the {100} plane showing the measurement results
by the EBSP method in thick steel plate used at FIG. 2. In the example
shown in FIG. 3, the black parts are {100} planes forming an angle of
±15° with respect to the plane vertical to the external stress.
The area ratio of the {100} planes is 14% at the posit-on 5 mm below the
surface of the steel material, 14% at a portion positioned at 1/4 of the
plate thickness from the surface, and 6% at a portion positioned at 1/2
of the plate thickness. Thick steel plate with a {100} area ratio of 30%
or less at the surface layer parts of the steel plate and 15% or less at
the center part of the steel plate in this way, as explained above,
exhibits a high crack arrestability of a Kca at -10° C. of 200
MPam0.5. Further, if observing the fracture surface of the test
piece, a shear lip of about 10% of the plate thickness was observed at
the surface layer parts.

[0039]The smaller the area ratio of the {100} planes, the better the crack
arrestability, but if extremely small, the other texture grows and
anisotropy ends up occurring in the crack arrestability, so the ratio for
the steel plate surface layer parts is preferably 5% or more and for the
steel plate center part is 3% or more.

[0040]The above effect of improvement of the crack arrestability is
particularly remarkable in steel plate with a yield stress of 390 to 500
MPa and steel plate with a plate thickness of 40 to 100 mm. The reason is
that in the region where the yield stress is less than 390 MPa or over
500 MPa and the plate thickness is less than 40 mm or over 100 mm, it is
difficult to form a distribution where the crystal grain size or texture
differ in the plate thickness direction such as prescribed in the present
invention.

[0041]Below, the reasons for limiting the amounts of the elements will be
explained.

[0042]C has to be 0.03% of more to secure the strength and toughness of
the thick steel plate. This is the lower limit. Further, if C exceeds
0.15%, it is difficult to secure a good HAZ toughness, so this becomes
the upper limit.

[0043]Si is effective as a deoxidizing element and strengthening elements,
so 0.1% or more is necessary, but if over 0.5%, the HAZ toughness greatly
deteriorates, so this is the upper limit.

[0044]Mn has to be 0.5% or more so as to economically secure strength and
toughness of the thick-gauge matrix material. However, if Mn is added
over 2.0%, the center segregation becomes remarkable. The matrix material
at this part and the HAZ toughness deteriorate, so this is the upper
limit.

[0045]P is an impurity element and has to be reduced to 0.02% or less to
stably secure the HAZ toughness.

[0046]Further, S is also an impurity element and has to be reduced to
0.01% or less to stably secure the characteristics of the matrix material
and HAZ toughness.

[0047]Al functions for deoxidation and is required for reducing the
impurity element O. In addition to Al, Mn and Si also contribute to the
deoxidation, but even if these elements are added, if 0.001% or more of
Al is not present, it is difficult to stably suppress O. However, if Al
is over 0.1%, alumina-based coarse oxides and their clusters are formed
and the matrix material and HAZ toughness are impaired, so this is made
the upper limit.

[0048]Ti is important in the present invention. By adding Ti, TiN is
formed and it is possible to keep the austenite grains from becoming
larger in size at the time of heating the steel slab. As explained above,
if the austenite grain site becomes larger, the grain size of the bainite
after the transformation also becomes larger, so to obtain the necessary
size of the bainite grains, Ti has to be added in an amount of 0.005% or
more. However, excessive Ti addition invites a drop in the HAS toughness
due to the formation of TiC, so 0.02% was made the upper limit.

[0049]Ni is the most important in the present invention. By controlling
the amount of addition of Ni to a suitable value in this way and
controlling the cooling rate in the process of cooling the steel plate,
in the above way, the subunits of the bainite, that is, the crystal
grains when defining the boundary where the crystal misorientation angle
is 15° or more as the grain boundary, can be made finer. To
exhibit this effect, the amount of angle of Ni has to be 0.15% or more.
However, Ni is an expensive element. Excessive addition is costly.
Further, there is also an upper limit to the effect of addition of Ni, so
2% is preferably made the upper limit.

[0050]N is important in the present invention. As explained above, TiN has
to be formed in the steel material, so 0.001% is made the lower limit. On
the other hand, if the amount of addition of N becomes excessive,
embrittlement of the steel material is incurred, so 0.008% is made the
upper limit.

[0051]Further, in addition to the above added elements, by mass %, one or
more of Cu: 0.1 to 1%, Cr: 0.1 to 1%, Mo: 0.05 to 0.5%, Nb: 0.005 to
0.05%, V: 0.02 to 0.15%, and B: 0.0003 to 0.003% may be included as
chemical components. By adding these in the lower limits or more, the
strength and toughness of the matrix material are secured. However, if
these elements are too great, the HAZ toughness and weldability fall, so
it is necessary to set upper limits to these elements.

[0052]Further, in addition to the above added elements, one or more of, by
mass %, Ca: 0.0003 to 0.005%, Mg: 0.0003 to 0.005%, and REM: 0.0003 to
0.005% may be included as chemical components. By adding these, the HAZ
toughness is secured.

[0053]Next, a preferable method of production of high strength thick steel
plate of the present invention will be explained. First, molten steel
adjusted to the above suitable chemical components is produced by a known
steelmaking method such as a converter and made into a steel material,
that is, a cast slab, by continuous casting or another normal casting
method. During the cooling at the time of casting or after the cooling,
the steel slab is heated to a temperature of 950 to 1250° C. to
make a single austenite phase. If this is performed at less than
950° C., the solubilization is insufficient, while if over
1250° C., the heated austenite becomes extremely coarse in grain
size, obtaining a fine structure after rolling becomes difficult, and the
toughness falls. This heated steel material may be rolled by
recrystallization rolling at 900° C. or more for the purpose of
making the austenite finer or may be left without rolling by
recrystallization rolling. Next, finishing rolling is used to create
steel plate of a predetermined thickness. After rolling, this is water
cooled. At this time, the steel is preferably rolled at a temperature of
670° C. to 850° C. by a cumulative rolling rate of 30% or
more and started to be cooled from a temperature of 650° C. or
more. The cooling rate at this time is preferably 25° C./sec or
more at the surfaces of the steel plate and 5° C./sec or more at
the center part of the steel plate. Further, sometimes water cooling is
switched to air cooling from a temperature of 500° C. or less for
the purpose of self tempering. Further, in accordance with need, after
cooling, the plate may be tempered and heat treated at a temperature of
300 to 650° C. to adjust the strength and toughness of the matrix
material. In this way, ultralow temperature rolling and complicated heat
treatment processes are not required, so the high strength thick steel
plate according to the present embodiment can be produced with a high
productivity and by a low cost. Further, the residual stress is also
suppressed, so the increase in cost due to the correction of the shape
can also be suppressed. This is therefore preferable.

[0054]As explained above, according to the present embodiment, by making
the amount of addition of Ni a suitable value to make the crystal grain
size of the mainly bainite structure finer and by forming a distribution
of texture reducing the area ratio of the {100} planes oriented to a
plane vertical to the loading direction, a high strength thick steel
plate can be improved in crack arrestability. Further, in steel plate
having a yield stress of 390 to 500 MPa and a plate thickness of 40 to
100 mm, the Kca at -10° C. showing the crack arrestability can be
made 170 MPam0.5 or more. Further, the productivity can be raised
and the cost lowered.

EXAMPLES

[0055]In the steelmaking process, the chemical components of molten steels
were adjusted, then the steels were continuously cast into cast slabs.
The cast slabs were reheated and further rolled to obtain thick steel
plates of thicknesses of 40 to 100 mm which were then water cooled. At
this time, part of the steel plates were air cooled (comparative
examples). After this, in accordance with need, the plates were heat
treated to produce thick steel plates of yield strengths of 390 MPa to
500 MPa. Table 1 shows the chemical components of the thick steel plates.

[0056]The thick steel plates were measured for microstructure phase
fractions, mechanical properties, average crystal grain size, and crack
arrestability. Among these, as the microstructure phase fractions, an
optical microscope was used to photograph the microstructures at a
position 5 mm below the surface of the plate thickness and positions at
1/4 and 1/2 of plate thickness by a power of X400, then image analysis
was used to find the average value of the area ratios of the different
phases with respect to the measured full field regions at the different
positions. Further, as the yield stress (YS) and tensile stress (TS), the
average values of two test pieces were found. Further, as the Charpy
absorbed energy (vE-40) at -40° C., the average value of three
test pieces was found. Further, the average crystal grain size was found
by using the EBSP (Electron Back Scattering Pattern) method to measure
500 μm×500 μm regions at 1 μm pitch, preparing a map of
grain boundaries with a crystal Disorientation angle with adjoining
grains of 15° or more, and finding the circle equivalent diameter
of the crystal grains at that time by image analysis. Further, the
measured EBSP data was used for analysis of the crystal direction, a map
of {100} planes forming an angle of ±15° with respect to the
plane vertical to the loading direction was prepared, and the area ratio
with respect to the total field region was found by area ratio. Note that
the measurement positions of the average crystal grain size and area
ratio of the {100} planes are positions about 10% of the plate thickness
below the surface of the thick steel plate (below referred to as the
"surface layers") and the center part of the plate thickness (below
referred to as the "center"). Further, the crack arrestability was tested
by a temperature gradient type standard ESSO test (original thickness and
plate width of 500 mm respectively). The measurement results of the thick
steel plates are shown in Tables 2 and 3 together with the methods of
production.

[0057]Steels 1 to 8 satisfy the requirements of the present invention in
chemical components and crystal grain size, so had Kca at -10° C.
showing the crack arrestability of superior values of 170 MPam0.5 or
more. In particular, Steels 1 to 6 satisfy the requirements of the
present invention in {100} area ratio, so exhibited superior values of
195 MPam0.5 or more. Further, they exhibit mainly bainite
microstructures and have as mechanical properties yield strengths (YS) of
395 to 480 MPa and tensile strengths (TS) of 530 to 640 MPa--all high
values.

[0058]As opposed to this, steels 9 and 10 have amounts of addition of Ni
of 0% and 0.1% or lower than the lower limit of the present invention. As
a result, the crystal grain size at both the surface layer and the center
part is over the upper limit of the range of the present invention.
Further, Steel 9 has an {100} area ratio at the surface layer parts over
the upper limit of the range of the present invention. For this reason,
they exhibited a Kca at -10° C. of a low value of 80 to 95
MPam0.5.

[0059]Further, Steel 11 has chemical components satisfying the present
invention requirements, but has a crystal grain size and {100} area ratio
at the surface layer parts over the upper limit of the range of the
present invention. For this reason, it exhibited a Kca at -10° C.
of a low value of 75 MPam0.5.

[0060]Further, Steel 12 does not satisfy the requirements of the present
invention in the Ti of the chemical components, so the crystal grain size
is over the upper limit of the range of the present invention at the
surface layer parts. Further, it has an {100} area ratio at the center
part over the upper limit of the range of the present invention. For this
reason, it exhibited a Kca at -10° C. of a low value of 120
MPam0.5.

[0061]Further, Steel 13 satisfies the requirements of the present
invention in chemical components and crystal grain size of the surface
layer parts, but has a crystal grain size of the center part higher than
the upper limit of the present invention For this reason, even if it
satisfies the requirements of the present invention in the {100} area
ratio, the Kca at -10° C. becomes 150 MPam0.5 and a high
crack arrestability could not be exhibited.

[0062]From the above embodiments, it was confirmed that by application of
the present invention, high strength thick steel plate superior in crack
arrestability having a yield stress of 390 to 500 MPa, having a plate
thickness of 40 to 100 mm, having a structure mainly comprised of
bainite, and having a Kca at -10° C. of 170 MPam0.5 or more
can be provided.

[0063]Further, the present invention is not limited to the above
embodiments and can be carried out changed in various ways in the range
not deviating from the main gist of the present invention.

INDUSTRIAL APPLICABILITY

[0064]The present invention can provide thick steel plate superior in
crack arrestability, high in yield stress, and having a plate thickness
of 40 mm or more at a low cost and can meet the demands for safety and
lower cost in shipbuilding, tanks, buildings, and other large sized
structures, so has great industrial applicability